I've had the most success in a check/double-check set up. Most people will tell you that it's labor intensive, and they're not wrong. However, I suspect that those people have never had to manually change two components on a thousand boards at a shot.
Print out your machine loading list. Have one operator load the parts. Have another operator (or QC person) double check that the parts are loaded per the load sheet. Run boards with abandon.
Another possible solutions involve reviewing the component selection with the customer, and finding out if these two different parts could be combined into a single part (maybe use the higher voltage rated part in place of the lower rated part); that way, you're only loading one part on the machine. Might have the added advantage of driving the component cost down as you will now need to buy twice as many of the same part.
Cheers, ..rob
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